1. Field of the Invention
The present application generally relates to image sensors made in monolithic form and used in digital image acquisition devices such as cameras and photographic devices. The present application relates to such devices, whether they are autonomous or incorporated in a device having other functions such as, for example, a portable phone or an automobile, air or water vehicle during a night travel or in adverse weather conditions.
The present application more specifically relates to a so-called high dynamic range image sensor implementing a compression method.
2. Discussion of the Related Art
A pixel of an image sensor essentially comprises a reverse-biased photodiode having its junction capacitance discharged by a photocurrent as a function of a received light intensity. The lighting level received by a pixel is determined by measuring the voltage across the photodiode at selected times among which the end of a so-called image acquisition or integration period, before and after which the pixel is reset by recharging its photodiode. The decrease in the voltage across the photodiode during the acquisition, that is, the intensity of the photocurrent having discharged its capacitance, is proportional to the light intensity. For high light intensities, the photocurrent may be such that the discharge of the junction capacitance during the acquisition phase adversely affects the measurement. In particular, beyond a given light intensity threshold, which depends on the integration time and on the features of the pixel elements, the photodiode reaches before the end of the integration time a so-called saturation discharge level and luminosity differences are no longer discriminated. To increase the discrimination of light areas of strong intensity, the pixel has been provided to be controlled during part of the integration time period, generally the second half thereof, to enable compression of the signal. A sensor using such a compression enables obtaining images having a decreased contrast, which enables covering an increased lighting range and is called a high dynamic range sensor.
However, whatever the imager synchronization mode, imagers using pixels with four transistors, in which a transfer transistor is interposed between the cathode of the photodiode and a read node, have unexpected malfunctions. In particular, saturations of the sensors can be observed while the compression should enable pushing back the saturation limit.
The above expression “image synchronization mode” applies to rolling shutter devices and to global shutter devices. In a rolling shutter device, each pixel line is successively exposed, the integration times of each line being offset with respect to one another. In a global shutter device, all sensor pixels are exposed simultaneously. A global shutter device preferably uses pixels with four transistors.
A rolling shutter device could indifferently use pixels with four transistors or pixels with three transistors which comprise no transfer transistor. However, due to the previously-described malfunctions, rolling shutter imagers preferably use pixels with three transistors.
However, imagers with three transistors exhibit various unwanted effects which do not exist with imagers with four transistors. Especially the photodiodes used in a pixel with three transistors exhibit a high reverse current (or dark current) which, further, significantly increases as the temperature increases. This dark current is linked to the structure of the photodiodes and especially to the presence of many interfaces with insulators in the charge storage area. Pixels with four transistors however use photodiodes in which the reverse current is considerably decreased, especially since the charges are stored in an area having its volume limited by non-insulating regions.